Manufacture of polyamide structures



Patented July 3, 1945 MANUFACTURE OF POLYAMIDE STRUCTURES William Way Watkins, Buffalo, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application April 23, 1942, Serial No. 440,211

11 Claims. (Cl. 18-48) This invention relates to an improved process for transparentizing structures composed of synthetic linear polyamides. For convenience, the invention will be discussed with particular reference to the transparentizing of synthetic linear polyamide sheets and films. It is to be understood, however, that the invention is not so limited.

The production of transparent synthetic linear polyamide sheets and films has been known prior to the present invention; however, such previously known processes have certain disadvantages. For example, the production of transparent synthetic linear polyamide films by a process of melt casting is disclosed in Foster U. S. Patent No. 2,212,770. The melt casting of films of any substantial width, however, presents innumerable problems. Difficulties have consequently been encountered in the commercial exploitation of such a process.

The copending application of Orion E. Schupp, Jr., Serial No. 362,660, filed October 24, 1940, discloses a method of dissolving synthetic linear polyamides in solutions of metal salts in alcohols. Such solutions can be readily used for the extrusion and casting of films and sheets. However, by the casting of films and sheets in accordance with the above-identified Schupp process, extruded or cast sheets and films will be opaque in nature. The opaoueness is due to the presence of voids'within' the sheet or film structure. Sheets and films produced from other solutions of syntheticlinear polyamides will also often be opaque due to the presence of voids therein. For certain purposes, it is highly desirable that sheets and films composed of synthetic linear polyamides be clear and transparent.

It is an object of this invention to provide a new and improved process for the transparentizing of synthetic linear polyamide structures which are opaque by reason of the presence of voids.

Other objects of the invention will ,appear hereinafter.

By the term, synthetic linear polyamides, as used throughout the specification and claims, is meant synthetic linear polyamides of the types described in U. S. Patents Nos. 2,071,250, 2,071,- 253 and 2,130,948. These polyamides contain amide groups where R is hydrogen or a monovalent hydrocarbon radical) as an integral part of the main chain of atoms in the polymer. Further, the average number of carbon atoms in the segments of the chain separating the amide groups in these polyamides is at least two.

In accordance with the present invention, synthetic linear polyamide structures having a quench index above 125 mils and which are opaque by reason of the presence of voids can be transparentized by subjecting said structures,

1 in their water-wet state (i. e. containing the amount of water which the structure willv absorb when in equilibrium with water in the liquid phase), to treatment with a non-aqueous liquid which is non-reactive with and incapable of completely dissolving said polyamide at the temperature of treatment, said treating liquid having a temperature of at least 75 C. but below, but not exceeding 10 below, the temperature at which said liquid will completely dissolve or melt said 20 polyamide; that is, substantially destroy the said polyamide structure. By the term "completely dissolve or melt or by the term substantially destroy the polyamide, as used in the present specification and claims, is meant the dissolving,

melting or destruction of a self-supporting film of the polyamide, under the conditions of treatment, to the point where it is no longer selfsupporting,

The following specific examples illustrate certain preferred methods for carrying out the present invention, it being understood that the invention is not to be limited to the details set forth in these examples:

Example I Five hundred parts of an interpolymer prepared from hexamethylene diammonium adipate and hexamethylene diammonium sebacate in the proportion of 30% of the former and of the 4 latter are dissolved in a solution comprising 500 parts of calcium chloride and 1500 parts of methanol. A glass plate is coated with a quantity of the solution and excess solution is removed by means of a doctor knife. The methanol is removed completely by evaporation and the plate containing the film is washed in tap water to "remove the calcium chloride therefrom. The resulting film is opaque by reason of the presence of voids therein, and of little interest where 50 it is desired to have a transparent film. The

film, while still wet, is dipped in a bath of glycerol, maintained at a temperature of to C. The film remains immersed in the glycerol for only a period of about three seconds. After removal from the bath, the film is washed to reficient tension to prevent the formation of wrinkles in the film. The resulting film will be characterized by glass-clear transparency and very satisfactory physical properties.

Example II Afilm produced in accordance with the pro-- cedure of Example I which has been washed free of calcium chloride and which has not yet been dried is dipped into a bath of white oil maintained at a temperature of 140 to 145 C. for a few seconds. After removal from the bath, the film is washed with suitable solvents to remove the white oil, and it is then dried in the customary manner. The film has good transparency and satisfactory physical characteristics.

Example III A film produced in accordance with the method of Example I and which has not yet been dried is dipped for a period of a few seconds into a bath of aniline having a temperature of between 90 and 100 C. After removal from the bath, the film is washed in methanol and dried while maintained under sufilcient tension to prevent the formation of wrinkles in the film. The resulting film will have glass-clear transparency, but will be slightly brittle in-nature.

For the production of transparent synthetic linear polyamide articles in accordance with the present invention, it is preferred to use an interpolyamide prepared from hexamethylene diammonium adipate and hexamethylene diammonlum sebacate, preferably in the proportion of 30% of the former and 70% of the latter. It is nevertheless possible to use interpolymers containing the above two ingredients in a range between 15% of the former and 85% of the latter to 45% of the former and 55% of the latter. The above range represents the most readily meltable interpolymers which can be prepared from the above two compounds.

Although the above-named interpolymers are particularly useful for the purpose of the present invention, it is within the scope of the invention to use any synthetic linear polyamide, including synthetic linear interpolyamides, provided that the polyamide has a quench index above 125 mils, and provided that it is film-forming or fiberforming in character. Whether a synthetic linear polyamide has a quench index of above 125 mils can be determined in the following manner: Sheet aluminum of 0.008 inch thickness is wrapped around a steel wedge having a width of approximately 1% inches, a length of approximately 8 inches and an included angle of 25. The seams of the aluminum sheet are locked and hammered so that a water-tight mold is constructed. This mold is filled with flakes of the synthetic linear polyamide to be tested, and the solved from the polyamide, for example, by means of sodium hydroxide solution. It is found that the polyamide at the tip of the mold and for some distance upward is transparent because of rapid quenching. The quenching properties of the 0 move the glycerol and it is then dried under sufpolyamide determine how much of the wedge will be clear andhow much will be cloudy or opaque because of crystallization. The line of demarcation between clear and cloudy polyamide can be easily seen and marked with ink. The thickness of the wedge at various points along this line is measured with a micrometer. An average of four such measurements is usually sufficient and is taken as the quench index of the polyamide. The quench index of a given polyamide can be reproduced within about :5% of a given value.

In the above examples specific reference was made to the use of synthetic linear polyamides dissolved in calcium chloride-methanol solvents. Numerous other metal salt-alcohol solvents may be used in the same manner. The above-identified Schupp application discloses other metal salts which may be used as equivalents for calcium chloride. For example, zinc, magnesium, lithium, strontium, aluminum, titanium, bismuth. cobalt and iron, bromides, iodides, nitrates and thiocyanates may be used. The concentration of poh'mer in the solvent may vary between wide limits. It is only necessary that the .solution may be coated to produce a film or may be spun to form a thread, or formed in the shape of other articles. It is preferred, however, to use solutions comprising substantially saturated solutions of the polyamide, and it is also preferred that the metal salt concentration used be that which will give substantially the highest concentration of polymer in the solution. Synthetic linear polyamide structures which are opaque by reason of the presence of voids will also be roduced by numerous other known methods for the production of synthetic linear polyamide structures; for example, the production of such structures from alcohol solutions of these polyamides or solutions of these polyamides in other solvents. The present invention is applicable to the transparentizmg of structures of any of the above-mentioned synthetic linear polyamides which are opaque due to the presence of voids however produced.

The treatment of synthetic linear polyamide structures in accordance with the present invention may be carried out with any non-aqueous liquid which is non-reactive with and incapable of completely dissolving the polyamide at the temperature of treatment, the treatment temperature being at least 75 C. Besides the glycerol, wlnte oil and aniline described in the examples, one might mention such diverse materials as higher alcohols, for example, normal hexyl alcohol, Dowtherm heating fluid (a mixture of diphenyl and diphenyl oxide), and Wood's metal.

Although not a part of the present invention, it is believed that polyamide films and the like can be similarly transparentized by the use of certain highly concentrated salt solutions, such as may be heated to temperature of the order of 0. Such a salt solution may consist of an aqueous zinc chloride solution of sufiicient concentration to have a boiling point of at least 140 C.

The treatment is carried out at a temperature of at least 75 C. but below, but not exceeding 10 below, the temperature at which said liquid will completely dissolve or melt said polyamide. In other words, if the liquid used has no dissolving action on the polyamide even at elevated temperatures, it should be used at a temperature within 10 of the melting point of the polyamide. If, on the other hand the liquid will completely dissolve the polyamide at an elevated temperature above 75 0., it should be used at a temperature below, but not exceeding 10 below, that temperature at which it will completely dissolve the polyamide. Under no circumstances, however, should the treating liquid have a temperature or less than 75 C.

With regard to the tension used during the drying of the film, or similar articles, there is no objection to applying considerable tension to the film. It is desirable, however, that the film be held sufficiently taut to prevent the formation of wrinkles therein which would normally be caused by drying the film in a relaxed condition.

For convenience, this invention has been discussed with particular reference to the production of sheets or films. It is to be clearly understood, however, that the invention is not limited to the production of sheets or films, and that equally desirable results may be produced if articles of other forms are produced in accordance with the present invention. For example, the invention is applicable to the production of yarns, fibers, films, or/even structures having considerable mass such as rods, tubes, plates, and. .the like. Care must be taken, however, with more massive objects because by the time desirable results are accomplished with the interior portion oi the objects, an undesirable change may [be effected on the exterior portion.

The present invention provides for a simple, unique and practical process for transparentizing synthetic linear .polyamide structures. Films having a glass-clear transparency can be produced in accordance with the present invention, and such films can be produced without seriousl affecting the physical characteristics of the films.

Since it is obvious that many changes and modiflcationscan be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited except as set forth in the appended claims.

I claim:

1. The process of transparentizing a structure composed of a synthetic linear polyamide having a quench index above 125 mils and which is opaque'by reason of the presence of voids which comprises subjecting said structure, in its waterwet state (i. e., containing the amount of water which the structure willabsorb when in' equilibrium with water in the liquid phase), to treatment with a non-aqueous liquid which is non-reactive with and incapable of completely dissolving said polyamide at the temperature of treatment, said treating liquid having a temperature of at least 75 C. but below, but not exceeding below, the temperature at which said structure is completely destroyed.

' is opaque by reason of the presence of voids which but below, but, not exceeding 10 below, the temperature at which said structure is completely de- Stroyed. 4. The process of transparentizing a structure composed of a synthetic linear polyamide having a quench index above 125 mils and which is opaque by reason of the presence of voids which comprises subjecting a structure composed of an inter-polymer prepared from hexamethylene di- 2. The process of transparentizing astructure composed of a synthetic linear polyamide having a quench index above 125 milsand which is opaque by reason of the presence of voids which comprises subjecting a structure composed of a synthetic linear interpolyamide, in its water-wet state (i. e., containing the amount of water which the structure will absorb when in equilibrium with water in the liquid phase) to treatment with a non-aqueous liquid which is non-reactive, with and incapable of completely dissolving said polyamide at the temperature of treatment, said treating liquid having a temperature of at least 75 C. but. below, but not exceeding 10 below, the temperature at which said structure is completely destroyed.

3. The process of transparentizing a structure composed of a synthetic linear polyamide having a quench index above 125 mils and which ammonium adipate and hexamethylene diammonium sebacate in the proportion of 30% of the former to 70% of the latter, in its water-wet state (i. e., containing the amount of water which the structure will absorb when in equilibrium with water in the liquid phase), to treatment with a non-aqueous liquid which is non-reactive with and incapable of completely dissolving said polyamide at the temperature of treatment, said treating liquid having a temperature of at least 75. C. 'but below, but not exceeding 10 below, the temperature at which said structure is completely destroyed.

5. The process of 'transparentizing a film composed of a synthetic linear polyamide having a quench index above 125 mils and which is opaque by reason of the presence of voids which comprises subjecting said film, in its water-wet state (i. e., containing the amount of water which the structure will absorb when in equilibrium with water in the liquid phase), to treatment with a non-aqueous liquid which is non-reactive with and incapable of completely dissolving said poly-' amide at the temperature of treatment, said treating liquid having a temperature of at least 75 C. but below, but not exceeding 10 below, the temperature at which said structure is completely destroyed.

6. The process of transparentizing a film composed of a synthetic linear polyamide having a quench index above 125 mils and which is opaque by reason of the presence of voids which comprises subjecting a film composed of a synthetic linear interpolyamidain its water-wet state (i. e., containing the amount of water which the structure will absorb when in equilibrium with water in the liquid phase), to treatment with a non- "aqueous liquid which is non-reactive with and incapable of completely dissolving said polyamid at the temperature of treatment, said treating liquid having a'temperature of at least 75 C. but

below, but not exceeding 10 below, the temperature at which said structure is completely destroyed.

7. The process of transparentizing a film composed of a synthetic linear polyamide having a absorb when in equilibrium with water in the liquid phase), to treatment with a non-aqueous liquid which is non-reactive with and incapable of completely dissolving said polyamide at the temperature of treatment, said treating liquid having a temperature of at least 75 C. but below, .but not exceeding below, the temperature at which said structure is completely destroyed.

8. The process of transparentizing a film composed of a synthetic linear polyamide having a quench index above 125 mils and which is opaque by reason of the presence of voids which comprises subjecting a film composed of an interpolymer prepared from hexamethylene diammom'um adipate and hexamethylene diammonium sebacate in the proportion of of the former to of the latter, in its water-wet state (i. e., containing the amount of water which the structure will absorb when in equilibrium with water in the liquid phase), to treatment with a nonaqueous liquid which is non-reactive with and incapable of completely dissolving said polyamide at the temperature of treatment, said treating liquid having a temperature of at least C. but below, but not, exceeding 10 below, the temperature at which said structure is completely destroyed.

9. The process of transparentizing a structure composed of a synthetic linear polyamide having a quench index above mils and which is opaque by reason of the presence of voids which comprises subjecting said structure, in its waterwet state (i. e., containing the amount of water which the structure will absorb when in equilibrium with water in the liquid phase), to treatment with glycerol having a temperature between and C.

10. The process of transparentizing a structure composed of a synthetic linear polyamide having a quench index above 125 mils and which is opaque by reason of the presence of voids which comprises subjecting said structure, in its waterwet state (i. e., containing the amount of water which the structure will absorb when in equilibrium with water in the liquid phase), to treatment with white oil having a temperature between 140 and 145 C.

11. The process of transparentizing a structure composed of a synthetic linear polyamide having a quench index above 125 mils and which is opaque by reason of the presence of voids which comprises subjecting said structure, in its waterwet state (i. e., containing the amount, of water which the structure will absorb when in equilibrium with water in the liquid phase), to treatment with aniline having a temperature of 90 to 100 C.

WILLIAM W. WATKINS. 

